VHF and UHF are supposed to be line-of-sight, limited to roughly the visual horizon and not much more. Then, occasionally, a 2 meter signal shows up from 400 miles away, and the explanation is almost always tropospheric ducting.
How a temperature inversion creates a duct
Normally, air temperature and humidity decrease gradually with altitude, and radio waves bend only slightly as they pass through that gradient, which is why VHF and UHF signals are limited to roughly line of sight. Under certain weather conditions, particularly a temperature inversion where a layer of warm air sits above cooler air near the surface, that gradient becomes sharp enough to trap radio waves in a kind of atmospheric waveguide, a duct, that carries them far beyond the normal horizon with surprisingly little loss.
When to expect it
Tropospheric ducting shows up most reliably along coastlines and over large bodies of water, where stable, layered air conditions are common, and during specific weather patterns like high-pressure systems settling over a region for several days. Warm, stagnant summer weather is a classic setup, and some regions, particularly around the Great Lakes and along both US coasts, see ducting openings often enough that VHF/UHF DXers actively track weather conditions specifically looking for them.
Working a ducting opening
When ducting conditions set up, signals on 2m and 70cm, and sometimes even higher, can suddenly cover hundreds of miles, turning an ordinary evening into an unexpected DX opportunity on bands you’d never normally associate with distance work. Keeping half an ear on your local simplex frequencies and watching regional weather patterns for stable high-pressure setups is really the only preparation involved, since ducting openings tend to appear and close with little warning.
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